Depth measurements of a tool in a well bore are critical parameters. An oil well which traverses earth formations typically consists of a series of consecutively drilled and lined sections of borehole where each section is smaller in diameter than the next above section. The liner for each section typically is a tubular pipe string made up from lengths of steel pipe which are coupled to one another at tool joints. The lengths of steel pipe are more or less uniform. When a liner or liners are in the well bore, and cemented in place, the formation zones of interest are perforated to communicate with the well bore. In most instances, a production tubing with a production packer is utilized with the perforated liner. It is common to use one or more gas lift valves or other production devices disposed along the length of production tubing. Gas lift valves include plugs and chokes which are periodically manipulated by a slickline tool. The production string also can have downhole circulating and flow control devices.
Remedial and diagnostic operations conducted with a tool on a slickline typically involve a well tool passed through the production tubing string to the depth where the operation will be conducted. The operator locates the tool in the well bore at the desired depth by relying upon depth measurements obtained with a mechanical depth measuring device. The device utilizes a slickline wrapped around a measuring wheel and the revolutions of this wheel are correlated to depth and displayed on a recorder or depth gauge.
In logging open boreholes, most well tools for measuring well bore parameters are suspended and moved through the well bore on an electric wire line cable. The electric wire line is a composite structure containing electrical conductors in a core assembly which is encased in spirally wrapped armor wire. Depth of a well tool on an armored cable in a well bore is determined by utilizing predetermined magnetic marks located at regular intervals along the length of cable. The marks are detected by a magnetic mark detector and provide a basic depth measurement. The depth measurement is, however, typically adjusted for elongation due to cable stretch. In this type of system, the cable has an armored outer surface which is guided through tangential contact measuring wheels which tangentially engage the outer surface of the cable. Because of the cable construction, it cannot be wrapped around a measuring wheel and so there can be slippage in the wheel contact which results in an error in depth. U.S. Pat. No. 4,718,168 issued Jan. 12, 1988 illustrates and explains the effects of the various factors in depth measurement for an electric line cable.
Slickline well operations are quite different than electrical wireline operations. The slickline is a high quality length of wire which can be made from a variety of materials, (from mild steel to alloy steel) and typically comes in three sizes: 0.092; 0.108; and 0.125 inches in diameter. For larger sizes, a braided wire construction is utilized which is typically 3/16" or 7/32". The braided wire for all practical purposes has similar stretch characterized as a solid wire and is wrapped around the measuring wheel. Such braided wire is considered to be "slickline" herein. The slickline can be 10,000 feet or more in length. In a typical slickline operation, the speed of descent or ascend of the tool is at a high speed as compared to the speed of an electrical cable operation.
Heretofore, slickline depth measurements have only used a mechanical wheel measurement. Changes in tension in the slickline are monitored to indicate downhole tool engagement and disengagement.
The measuring wheel for a wheel measurement has a calibrated O.D. and the slickline is wrapped about or partially wrapped about the measuring wheel to prevent slippage. The measurement, however, obtained from a measuring wheel typically has an error factor in excess of 2 to 4 feet per 1,000 feet of length because of elastic stretch is not considered. Magnetic marks systems to measure stretch, such as disclosed in U.S. Pat. No. 4,718,168 have little application because magnetic marks, when placed on a slickline, quickly deteriorate as the wire is run in cased or completed wells.
A basic string of tools for a slickline operation typically includes a rope socket for connection to the end of the slickline, a weight or stem to assist descent into the wellbore by overcoming friction in a stuffing box or pack-off and the force of well pressure on the cross-sectioned area of the wire. Jars are also included (which can be mechanical or hydraulic) for providing downhole impact energy upon operation in a well bore. A universal or knuckle joint permits a flexible connection to the running or pulling tool to be used in the operations.
Pulling and running tools can also include battery operated service tools with recording memory means which collect fluid samples, measure pressure, measure temperature, gamma ray and flowmeter measurements, directional survey, diameter surveys and so forth as a function of time. With the present invention such tools can be correlated with respect to depth and time. Pulling and running tools are used in gas lift operations, downhole plugs and valve shifting, flow control devices, filling operations, pack off devices and so forth. In many situations, downhole profiles and landing nipples can be closely spaced to one another and difficult to locate depth wise. In other instances in the well, the location of perforations in the well bore or gas lifts and packers in the production tubing can be in error as to true depth. Heretofore, there has been no reliable way to determine any discrepancy in depth by use of a slickline.
In a slickline operation, the predominate factors affecting the depth measurement of a well tool suspended by a slickline are the elastic stretch, the temperature and the accuracy of the measuring wheel. When the well tool is moving, buoyancy and drag or lift affect the tension in the slickline and hence affect the depth measurement. The type of material and the cross-sectional area of the slickline will also affect the depth measurement.
A slickline as commonly acquired from a wire manufacturers will have a rated modulus of elasticity (sometimes called "stretch coefficient"). Stretch coefficients determined from short lengths of wire or slickline are not the same as the stretch coefficient for a long length of wire such as 10,000 to 15,000 feet. The stretch coefficient is also an average value and can vary from lot to lot of wire produced and is not specific to a particular slickline. Obviously, use of an inaccurate stretch coefficient in determining depth in deep wells can seriously affect the depth measurement.
The diameter of the measuring wheel will change in diameter under the effects of temperature. For example, in some environments a 70.degree. change in temperature can occur in a 24 hour period. Such temperature changes seriously affect the depth measurement with a measuring wheel. Temperature changes also affect the wire length. Also, over time, the measuring wheel will build up surface film which can increase the diameter of the wheel, or the wheel will wear which decreases the diameter. Any chance in the calibrated wheel diameter affects the depth measurement.
Buoyancy and the lift/drag factors can be a significant factor. In going in, the weight and the stuffing box friction largely determine the descent velocity through the liquid in the well bore. In coming out of a well bore, the operator can pull the tool as fast as the strength of the slickline will endure. In reaching the desired depth and reversing the direction of motion, the slickline tension changes considerably and thus the stretch of the slickline varies before the motion of the tool is affected.
In considering the present invention, it is entirely possible that technology will develop future electrical wireline cable which can be wrapped or partially wrapped about a measuring wheel. In such case such wireline should be considered "slickline". The principal feature in wrapping slickline with respect to a measuring wheel is to increase the area of contact and thus increase the friction between the slickline and the measuring wheel and eliminate slippage of the wireline relative to the measuring wheel. Slippage is an inherent defect in tangential contact measuring wheels.